In the pulping of comminuted cellulosic fibrous material, such as wood chips, in a continuous or batch digester the material is treated to remove entrapped air and to impregnate the material with cooking liquor while raising its pressure and temperature (e.g. to 150.degree. C. and 165 psi). Typically, the chips are steamed to purge them of air while simultaneously increasing their temperature, passed through air locks to raise their pressure, impregnated with heated cooking liquor, and then transported as a slurry to the digester.
In the past, in order to accommodate the purging, heating, pressurizing, and feeding functions, an apparatus is provided that is bulky, tall, and expensive. Normally a special building or super structure must be built to house or support this equipment. Such a building or super structure is built with structural steel and concrete, requires utilities, stairwells, and other accouterments, and contributes greatly to the cost of a continuous digester system. Also, the cost of the conveyor which transports chips to the inlet to the system is highly dependent upon the overall height of the system, which is typically on the order of about 115 feet for a digester which has a capacity of about 1,500 tons per day.
In U.S. Pat. No. 5,476,572 a method and apparatus are provided which utilize a delivery system which is much less massive, tall and expensive than the conventional systems. According to the present invention another variation of the approach taken in U.S. Pat. No. 5,476,572 is provided which also results in a greatly simplified chip feed system for a continuous digester, and also useful with batch digesters. According to the present invention a single generally vertical vessel, having a top and a bottom, is provided which performs the functions of a conventional chip bin, steaming vessel, and chip chute into a single vessel, typically smaller than the combined sizes of the other vessels (e.g., typically at least 20% smaller than the combined sizes), which results in a significant decrease in the cost of constructing and operating a chip feed system for a continuous digester. Even if the single vessel according to the invention is larger than corresponding prior art vessels, because of its simplicity, it is easier to construct, operate and maintain.
According to one aspect of the present invention a method of handling comminuted cellulosic fibrous material, such as wood chips, is provided to feed the material to a continuous or batch digester. The method comprises the steps of: (a) Confining comminuted cellulosic fibrous material in a predetermined, open, volume. (b) In the predetermined volume, establishing a first level of comminuted cellulosic fibrous material, and a second level, below the first level, of cooking liquor. (c) Subjecting the comminuted cellulosic fibrous material between the first and second levels to steam to effect steaming of the comminuted cellulosic fibrous material in the predetermined volume. (d) Slurrying the comminuted cellulosic fibrous material with cooking liquor below the second level, to produce a slurry in the predetermined volume. And (e) removing the slurry from the predetermined volume, further pressurizing the slurry, and feeding the pressurized slurry to a continuous or batch digester.
Preferably steps (a)-(e) are practiced substantially so continuously, comminuted cellulosic fibrous material being substantially continuously introduced into the predetermined volume from above the first level, substantially continuously flowing downwardly in the predetermined volume, and being substantially continuously removed from the predetermined volume below the second level. Steps (a)-(e) may also be practiced at superatmospheric pressure (e,g. about 0.1-4 bar). The slurry from step (e) may be fed directly to the top of a digester, or through an impregnation vessel.
According to another aspect of the present invention a digester system is provided comprising the following components: A continuous digester. A high pressure transfer device for transferring comminuted cellulosic fibrous material slurry under pressure to the continuous digester. A generally cylindrical vertically oriented vessel having a top and a bottom. Metering means for feeding comminuted cellulosic fibrous material into the vessel from the top thereof. Means for establishing a first level, of comminuted cellulosic fibrous material, in the vessel. Means for establishing a second, liquid, level in the vessel, the second level below the first level. Means for supplying steam to a volume between the top of the vessel and the second level. And means for withdrawing a slurry of comminuted cellulosic fibrous material in liquid from adjacent the bottom of the vessel and feeding the slurry to the high pressure transfer device.
The withdrawing means may comprise a combination of a metering screw feeder, or any type of conventional metering conveyor, and a low pressure pump, while the metering means may comprise a conventional chip meter such as sold by Kamyr, Inc. of Glens Falls, N.Y., a screw conveyor, or any other type of conventional metering conveyor.
The vessel may include, between the second level and the bottom of the vessel, a single convergence type configuration providing a reduced cross sectional area of more than 50% compared to the cross sectional area at the second level. A pressure isolation device is preferably provided between the metering means and the vessel to control the vessel pressure. The vessel may be operated pressurized or unpressurized.
According to yet another aspect of the present invention an upright vessel having a top and a bottom, is provided. The vessel takes the place of a conventional chip bin, steaming vessel and chip chute in a conventional system for feeding chips or like comminuted cellulosic fibrous material to a continuous digester, and preferably has a size of at least 20% less than the combined sizes of the conventional chip bin, steaming vessel and chip chute, and preferably is at least 50% smaller in size. Regardless of size, the vessel is less complex, making it easier to construct, operate and maintain. The vessel comprises: A first conduit for supplying steam to the vessel. A first automatically controlled valve in the first conduit. A temperature controller for sensing the temperature within the vessel and controlling the first valve at least partially in response thereto. A second conduit for supplying liquid to the vessel. A second automatically controlled valve associated with the second conduit. A level controller for sensing the level of liquid within the vessel and controlling the second valve at least partially in response thereto. Metering means for feeding comminuted cellulosic fibrous material into the vessel from the top thereof. And means for withdrawing a slurry of comminuted cellulosic fibrous material in liquid from adjacent the bottom of the vessel. The details of the components described above may be as set forth earlier.
According to yet another aspect of the present invention there is provided a method of simplifying the chip feed system of a continuous digester having a high pressure transfer device comprising steps of: (a) Removing the conventional chip bin, steaming vessel and chip chute from operative connection to the high pressure transfer device. (b) Replacing the chip bin, steaming vessel and chip chute with a single vertical vessel having steaming means and chip slurrying means associated therewith and having an outlet from the bottom thereof, the single vertical vessel preferably having a volume at least 20% less than the combined volumes of the chip bin, steaming vessel, and chip chute. And (c) connecting the outlet from the bottom of the single vertical vessel to the high pressure transfer device.
The invention also relates to a chips steam and feeding device for feeding steamed chips to a batch digester in an effective and simplified manner.
It is the primary object of the present invention to provide for the simplified, less expensive, yet effective feed of chips or like comminuted cellulosic fibrous material to a continuous or batch digester. This and other objects of the invention will become clear from an inspection of the detailed description of invention and from the appended claims.